THE OPEN-HEARTH FURNACE. 131
complete combustion and to have a slight excess of oxygen; moreover., the air enters cold, while the gas is generally warm; but in practice the relative volumes of the gas and air chambers will usually be determined more by the difficulties of getting room than by nice .calculations on the volumes of gases. It is well, however, to keep the principle in mind that if the gas is hot there is less work for the gas chamber to do, and the fact that the gases escaping to the chimney are at a high temperature has nothing to do with the case, for if the entering gases are hot the escaping gases must be hotter. With a given sized chamber, the escaping gases will be just a certain number of degrees hotter than the gases that go into it. If this difference is 300°, then if the entering gas is 400°, the escaping gases will be 700°, and if the entering gases are 700°, the outgoing gases will be 1000°, so that it would be useless to increase the size of the chamber just because the outgoing gases are hot, for these conditions are caused by hot entering gases, and the escaping products would be hot no matter how large the chamber might be. Different melters have different ideas as to how a furnace should be run, and it is sometimes better to let them have their own way than to- change the practice radically to accomplish a small saving. One melter may do better work if the air is extremely hot, while another may prefer that the air be colder than the gas. These differences also arise from the particular construction of ports, so that if an attempt is made to change the relative temperature of the chambers, it might necessitate a change in the construction of the ports and the roof of the furnace.
Under such circumstances the most practicable thing to do is to run the temperatures of the chambers in accordance with the construction of the ports and the roof. These conditions will oftentimes make considerable difference in the relative amounts of heat delivered to the gas and air chambers, and, therefore, will determine the relative size of the two chambers, and this may account for the difference of opinion concerning the proper area for the regenerators.
In the Schonwalder construction, introduced abroad, the main point is to have large flues underneath the checkers, so as to insure free draught in all parts of the chamber, so that the hot gases will go down and the cold gases come up, equally over the entire horizontal cross-section. To make more certain, the chamber is divided